Field of the Invention
The invention relates to an anti-theft system for a motor vehicle. It relates in particular to a system for locking the doors of a motor vehicle and to an immobilizer block, which is used to permit starting of the engine upon authorization.
A known anti-theft system (U.S. Pat. No. 4,918,955) has an ignition lock with a transmitting antenna in the form of a coil. The coil is excited by an oscillator. The ignition key has an oscillating circuit that cooperates with the transmitting coil. As soon as the ignition key is introduced into the ignition lock, encoded information is transmitted from the ignition key to the lock. If the encoded information matches command code information, a driveaway block in the motor vehicle is released so that the motor vehicle can be started.
In such systems, however, it is possible for no coded information to be detected by a receiving circuit, despite the insertion of a properly functioning ignition key. The reason therefor is that because of component tolerances or the effect of temperature, an operating point of the system has shifted so far that it is in a so-called null point.